my key fob locks all doors etc but does not unlockat all any ideas

Does your key have an LED indication when it's transmitting? If so, does it light up for both locking and unlocking? Unlock button might be busted.

Do you have a spare key? Does that work?

My remote stopped unlocking the Central Locking, but would still unlock the boot, and would lock the car.  This behaviour was repeated at the usual range of distances from the car that I normally operate the key fob.  The LED on the key fob did not appear to be flashing for any of the button presses.  The other, rarely used, key fob worked OK.  I changed the CR2032 battery in the fob, and without even needing to re-sync, the car unlocked on the next press of the unlock button, and this time, the LED flashed too.  

I checked the old battery voltage, which was 2.9V, which seems quite close to the nominal 3V published for a CR2032, but the new battery voltage seems to be nearer 3.3V.  I checked this with two multimeters.